Recently, there has dramatically been developed an electronic device which utilizes a III-V compound semiconductor mainly based on GaAs, taking advantage of features thereof such as abilities to operate at a ultra-high speed and at a higher frequency, and has still advanced steadily. When fabricating the electronic device which utilizes a compound semiconductor, a thin film crystalline layer with necessary properties has conventionally been fabricated on a single crystal substrate by means of various procedures such as an ion implantation method, a diffusion method, or an epitaxial growth method. The epitaxial growth method, among the above described various methods, has widely been used since it has become possible not only to control an impurity amount but also to control crystal composition or thickness in an extremely wide range and with precision.
Although procedures such as a liquid phase method, a vapor phase method, and a molecular beam epitaxy (referred to as a MBE method, herein after) which is one of the vacuum deposition methods have been known as the epitaxial growth method used for such purposes as described above, the vapor phase method has commercially and widely been used because of its ability to process a large amount of substrates with high controllability. Especially, an metalorganic pyrolysis method (referred to as a MOCVD method, hereinafter), in which an organometallic compound or a hydride of atomic species constituting an epitaxial layer is used as a source material and is pyrolyzed on the substrate to grow a crystal thereof, has recently been used widely since this method is applicable to a wide range of substances and is suitable for controlling the crystal composition and thickness with precision.
Based on the development of such manufacturing techniques as described above, there have recently been made various attempts to improve characteristics of a high electron mobility field effect transistor (referred to as a HEMT, hereinafter) which attracts attentions because of its usefulness as an important component of a high-frequency communication instrument. The HEMT is also referred to as a high electron mobility transistor, a modulation doped field effect transistor (MODFET), or a hetero-junction field effect transistor (HJFET), and an epitaxial structure used for the HEMT is characterized in that an electron supplying layer for supplying electrons and a channel layer through which electrons travel are separated from each other and play respective roles, and that a two-dimensional electron gas accumulated in the channel layer has a high electron mobility. An epitaxial substrate used for manufacturing the HEMT can be fabricated by employing a MOCVD method such that each of crystalline layers of GaAs and AlGaAs having necessary electron characteristics is laminated and grown on a GaAs substrate to obtain a required structure.
Although GaAs and AlGaAs materials have widely been used as materials for fabricating the above described devices since these materials with any compositions can match the lattice constants thereof with each other and allow for producing various hetero junctions while keeping good crystallinity thereof, it is also possible to grow a crystalline layer of InGaP by selecting an In composition such that the lattice constant thereof matches with that of GaAs. In this case, InGaP being lattice-matched with GaAs is known to have an In composition of 0.482 to 0.483 and a Ga composition of 0.518 to 0.517.
As for a III-V compound semiconductor material, InxGa(1-x)As (wherein 0<x<1) is extremely suitable as a hetero junction material for manufacturing the HEMT, because InxGa(1-x)As is excellent in its electron transporting characteristic and is also capable of significantly changing its energy gap in accordance with the In composition. However, the InxGa(1-x)As cannot be lattice-matched with GaAs, so that it has conventionally been impossible to obtain an epitaxial substrate for the HEMT with significant physical properties by using a InxGa(1-x)As layer.
Based on the subsequent development of techniques, it has been found that a reliable hetero junction can be formed without unfavorably inducing a decrease in crystallinity such as producing a dislocation even when a material with lattice misfit is used provided that the misfit is within a limit of elastic deformation, so that there has been made an attempt to practically use an epitaxial substrate which utilizes InxGa(1-x)As as a hetero junction material. Such limit values in the lattice misfit material are given as a function of composition and layer thickness, and in a material based on a InGaAs layer with respect to a GaAs layer for example, the limit value has theoretically been known to be represented by an equation as described in J. Crystal Growth, 27 (1974) p.118 and in J. Crystal Growth, 32 (1976) p.265, and this theoretical equation is known to be experimentally correct as a whole.
Thus, even in the case of an epitaxial substrate of a HEMT structure which utilizes a GaAs substrate, it has become possible to manufacture an epitaxial substrate having an InGaAs layer as a part thereof by using a strain layer within certain ranges of composition and layer thickness. For example, under the condition of usual crystal growth, it is possible to epitaxially grow an InxGa(1-x)As layer in which x=0.20 and whose layer thickness is about 13 nm without inducing a decrease in its crystallinity, and consequently, an epitaxial substrate including such an InxGa(1-x)As layer as a channel layer part of the conventional HEMT through which two-dimensional electrons flow is utilized for fabricating an electronic device which has a higher mobility and is excellent in a noise characteristic compared with the conventional device.
The HEMT, in which InxGa(1-x)As as a strain layer is used for the channel layer part through which two-dimensional electrons flow, is referred to as a pseudomorphic high electron mobility field effect transistor (a pseudomorphic-HEMT) (hereinafter, referred to as a pseuodomorphic-HEMT).
In addition, as described above, InGaP can be lattice-matched with GaAs provided that an In composition is selected, and therefore, in the phseudomorphic-HEMT, an InGaP layer can be epitaxially grown as an electron supplying layer or spacer layer thereof instead of using an AlGaAs layer. InGaP provides a high-performance HEMT, because InGaP has advantages that impurities are hardly incorporated therein during the epitaxial growth and the crystal purity can be favorably maintained compared with AlGaAs, and that a deep level referred to as a DX center is never created when silicon is doped during the formation of an n-type layer as in the case of AlGaAs. In addition, there has been reported that InGaP is advantageously used for manufacturing an electron device because the InGaP has a larger energy gap and a lower surface level compared with AlGaAs.
When various epitaxial growths are performed in order to form a pseudomorphic-HEMT structure including an InGaP layer and an InGaAs strain layer on a GaAs substrate, the crystal growth has to be controlled so as to precisely control a thickness of a thin crystalline layer to be formed on the order of several nanometers, however, as a result of recent technical improvements, an MBE method being excellent in the layer thickness controllability as well as an MOCVD method being excellent in the mass productivity can control the layer thickness with high precision, and consequently, it is now possible to obtain an epitaxial substrate of the HEMT having substantially favorable characteristics.
As described above, when an InGaP layer is used for an electron supplying layer or an electron supplying layer and a spacer layer of the pseudomorphic HEMT structure, it has been found to be difficult to efficiently confine two-dimensional electrons generated from the electron supplying layer to the InGaAs channel layer, although it is possible to achieve improvements in characteristic of the electronic device such as a temperature characteristic. Thus, it has been difficult to improve a current value of the electronic device by increasing the two-dimensional electron gas concentration or to reduce a transient resistance of the electron device by increasing the electron mobility.
The reason thereof has been considered that an energy band profile of InGaP is different from that of AlGaAs, that is, there is no difference between a position of a conduction band of the energy band structure of GaAs and that of InGaP. If there is no difference between these positions of the conduction bands, electrons generated from the electron supplying layer can not be efficiently confined to the InGaAs channel layer, and consequently, reductions in the two-dimensional electron gas concentration and in the electron mobility would be induced. As the preventative measures against such problems, Japanese Patent No. 3224437 discloses a constitution which improves the two-dimensional electron gas concentration and the electron mobility by inserting a strain InGaP space layer between a channel layer and an InGaP electron supplying layer in order to make a difference between the positions of conduction bands. In addition, Japanese Patent No. 2994863 discloses a constitution which improves the two-dimensional electron gas concentration and the electron mobility by inserting an AlGaAs spacer layer between a channel layer and an InGaP electron supplying layer.
However, comparing with a result reported with respect to an epitaxial substrate used for the conventional AlGaAs pseudomorphic HEMT structure in which an InGaAs layer is used as a channel layer, an n-AlGaAs layer is used as an electron supplying layer, and an i-AlGaAs layer is used as a spacer layer between the channel layer and the electron supplying layer, both constitutions disclosed in the above described Japanese Patent No. 3224437 and Japanese Patent No. 2994863 have not yet achieved satisfactory electron mobilities, in view of the capability of the epitaxial substrate used for the pseudomorphic-HEMT structure which can make characteristics of the electronic device favorable by increasing each value of the two-dimensional electron gas concentration and the electron mobility.
For example, further improvements are desired when the pseudomorphic HEMT structure epitaxial substrate is used for various portable instruments such as cellphones, because an on-resistance can be decreased by further improving the electron mobility and thus it is possible to reduce power consumption. In addition, further improvements in the electron mobility are also desired in view of capabilities to reduce a calorific value by lowering the power consumption and to miniaturize of an apparatus by further increasing a degree of integration. Thus, in the epitaxial substrate for pseudomorphic-HEMT structure which uses InGaP as an electron supplying layer or as an electron supplying layer and a spacer layer, a further improved epitaxial substrate is strongly desired which has a higher two-dimensional electron gas concentration together with a higher electron mobility compared with the currently reported results.